1. Field of the Invention
The present invention relates to a high precision chromatic dispersion measuring method that precisely measures chromatic dispersion values of the optical transmission line in an optical link system that uses an optical transmission line having chromatic dispersion such as an optical fiber, and then calculates a compensation amount for the measured chromatic dispersion. Furthermore, the present invention relates to an automatic dispersion compensating optical link system that automatically compensates chromatic dispersion in the optical transmission line and also polarization mode dispersion by controlling a dispersion compensator based on a compensation amount calculated using the above method.
This application is based on Patent Application No. 2001-198137 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In an optical link system that uses high-speed channels, degradation in the waveform is generated by the chromatic dispersion or higher order dispersion such as a dispersion slope of the optical fiber and the like forming the optical transmission line, and the degradation in the transmission quality which is created by the degradation is a problem. Specifically, the degradation is generated as a result of chromatic dispersion in the optical fiber or higher order dispersion thereof acting on the bandwidth of the optical signal causing the optical pulse waveform to breakdown, and allowing interference from adjacent optical pulses to be received. Moreover, because the chromatic dispersion of the optical fiber and the like also varies due to the ambient temperature, this also affects the stability of the transmission quality. Furthermore, a similar problem is also created when a chromatic dispersion is changed because of route switching when a fault such as a broken line occurs in a particular transmission section.
In order to solve these problems various chromatic dispersion measuring methods and automatic dispersion compensating optical link systems combined with dispersion compensation technologies have been proposed. One of these is a method in which the intensity of an optical clock signal extracted from received optical signals is monitored and the chromatic dispersion values are controlled such that the intensity is set either to the maximum or the minimum.
FIGS. 27A and 27B show the relationship between optical clock signal intensity and chromatic dispersion values for an 40 Gbit/s RZ signal generated using optical time division multiplexing (OTDM) and for a normal RZ signal with a duty ratio of 50% (cited document: G. Ishikawa and H. Ooi, “Demonstration of Automatic Dispersion Equalization in 40 Gbit/s OTDM Transmissions”, Technical Digest of European Conference on Optical Communication, pp. 519-520, 1998).
Because the optical clock signal intensity in the 40 Gbit/s RZ signal generated using OTDM shown in FIG. 27A has a local minimum value at the point when the eye opening is at maximum, the chromatic dispersion values are controlled such that the optical clock signal intensity is a local minimum value. In contrast, in the case of the RZ signal with a duty ratio of 50% shown in FIG. 27B, the chromatic dispersion values are controlled such that the optical clock signal intensity shows the maximum value.
In the above described conventional method, a simple correlation is assumed between the optical clock signal intensity and the eye-opening, and makes use of the characteristic that when the optical clock signal intensity becomes a local minimum value in an OTDM signal, and when the optical clock signal intensity reaches maximum in an RZ signal, the chromatic dispersion amount of the optical signal is zero and the eye opening reaches the maximum.
However, this is only true in the case where the incident light power into the optical fiber is sufficiently small and phase modulation caused by non-linear optical effects can be negligible. In contrast, in an actual transmission system there is practically no way to carry out the transmission under the above conditions and it is not possible to disregard the influence of non-linear optical effects such as cross phase modulation in wavelength division multiplexing and four-wave mixing, and self phase modulation effects. Namely, in a transmission medium such as an optical fiber, self phase modulation is generated by the optical non-linearity of the transmission medium itself, and due to the interaction of this self phase modulation and of the chromatic dispersion it cannot be guaranteed that zero dispersion will be the optimum dispersion value. In such cases, there is no guarantee that the point where the local minimum (maximum) clock signal intensity and the eye opening is at maximum will match the point where the chromatic dispersion is at zero. Namely, in the conventional technology it is difficult to achieve a sufficient suppression in transmission quality degradation using chromatic dispersion compensation.
Moreover, as described above, because the optimum dispersion value is obtained when the optical clock signal intensity is at the local minimum value and at the maximum value for an OTDM signal and RZ signal, respectively, the control method differs greatly in accordance with the encoding format. Furthermore, although in an OTDM signal the optical clock signal intensity is at the local minimum value when the eye opening is at maximum, two local maximums exist in the vicinity of the local minimum value. Therefore, to keep the optical clock signal intensity automatically to the local minimum value is extremely difficult. Moreover, as the maximum of the optical clock signal intensity is 0.8 and the minimum thereof is 0.4, the dynamic range thereof is only 3 dB. Accordingly, it is also difficult to perform stable control from the viewpoint of the SN ratio as well.
Furthermore, in the conventional technology, when monitoring the optical clock signal intensity in accordance with the chromatic dispersion, the optical clock signal intensity changes in the same way regardless of whether the chromatic dispersion shifts to the positive side or to the negative side, and the direction of the shift of a chromatic dispersion value is unknown. Therefore, in order to determine that direction, it is necessary to intentionally generate degradation in the eye opening by shifting the chromatic dispersion value. Namely, because it is not possible to avoid the degradation in the transmission quality caused by the degradation in the eye opening, monitoring the optical clock signal intensity while transmission is being performed has been difficult.